As many of you know, a few members and I have been doing quite a bit of research/measurements/calculations on the M3 suspension. Here is one finding which I thought should be explicitly expressed for the make benefit of the M3 community.

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Here is why lowering springs may ruin your M3's ride and handling:
(I will try to explain in the simpliest words possible)

The M3 front suspension has about 5.75" of total suspension travel from fully extended (i.e. on a lift) to fully compressed (let's assume no bump stop for now). About half of the 5.75", or 2.875", is dedicated to compression.

Now, let's take lowering spring brand A for example, which drops the car by 1" in the front. So 2.875"-1"=1.875", which means that you will still have 1.875" of compression travel left, which is ok.

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Well, guess what, there is a OE bump stop on the front suspension which is about 2.375" long uncompressed, and with a 1" drop, you would already be riding on it.

The bump stop behaves progressively (see sample figure below), meaning the rate increases with more compression. And when this bump stop compresses to about 1" (from 2.375"), its rate shoots up, adding significant stiffness to the total effective spring rate. At this point the bump stop is almost like a hard piece of rubber in comparison to the rest of the system.

Now, we were left with 1.875" of compression travel after the drop. Subtract another 1" due to the bump stop, and one is left with 0.875" of effective compression travel. (yes, it is about the width of a finger)

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The short compression travel (in this type of set up) does a few things:

1) You will get a very bumpy ride, especially when you go over imperfections, as the suspension is constantly hitting the compression limit.
2) This will constantly overwhelm the OE shocks, which is not designed to operate in such a high variable effective spring rate all the time.
3) Unpredictable handling. More weight is transfered to the wheel that has a stiffer rate. When a lot of weight is transferred to the front outer wheel when you are turning, you get understeer.

Hope this helps. This is not to say that all lowering springs will behave this way, but obviously some should be avoided if you don't want to sacrifice the ride and handling just for the look.

Correct me if I am wrong, but doesn't the Eibach's spring rate derive very closely to the AC Schnitzer spec? As I understand it, Eibach produces the springs for ACS to their specs, and the Eibach branded ones are very close in spec.?

I was looking into buying some Eibach springs. Would you recommend these or will they push the limit of the stock setup? I know that Dinan includes a shorter bumpstop and Eibach does not. I assume that Eibach believes that a shorter bumpstop is not necessary for their springs, and by adding one it will alter the overall spring rate.

In all seriousness, I really haven't experienced any negatives with my set of H&R springs (other than scratches under my bumper). I did one track day prior to installing the springs and I've done several since with no dramatic difference in the handling. Having said this I am planning to go with a set of coil-overs sometime this year.

I think coilovers is the best way to do it..you will improve the looks, you can adjust the height whenever you want(depending on the coilovers other things like compression too) and you improve the handling. Besides, you do not have to worry about things this quite interesting and informative thread talks about

I was looking into buying some Eibach springs. Would you recommend these or will they push the limit of the stock setup? I know that Dinan includes a shorter bumpstop and Eibach does not. I assume that Eibach believes that a shorter bumpstop is not necessary for their springs, and by adding one it will alter the overall spring rate.

Thanks...

I remember reading that the Eibach chief engineer said that the stock bump stop characteristics were factored into te design of their springs. There's no need to cut the bump stops with Eibach.

Really interesting thread, credit to the OP for such an excellent post.

I had coils on my 335i and definitely feel like my M3 could use a drop in the looks department. However, I'm loathed to mess with the suspension - I feel like the handling is awesome and just don't want to disrupt it. I was thinking about Eibachs but this thread (and a few others) are definitely pushing me towards staying stock.

I have H&R and felt a dramatic difference in handling on the Nordschleife. Alot less body roll! sharper turn ins! better felling at the limit. The stock setup is for M3 drivers that will never leave the public roads!